THE ROLE OF AUTOPHAGY IN AMIODARONE-INDUCED THYROTOXICOSIS PATHOGENESIS: A TRANSLATIONAL STUDY

Background: Amiodarone-induced thyrotoxicosis (AIT) is a challenging condition associated with high morbidity and mortality rates. The pathogenesis of the most demanding AIT type (i.e. type 2 AIT, AIT2) is not fully understood, and is currently mostly attributed to the cytotoxic effect exerted by amiodarone and its catabolites on thyrocytes. However, amiodarone acts as an autophagy inducer in in vitro settings different from thyroid cell cultures, but this aspect has not been studied on thyroid cells to date and we do not know whether there might be a role for autophagy in amiodarone-induced thyroid toxicity. Methods: The study included both an in vitro and an in vivo setting. In vitro experiments were performed on primary thyroid cells cultures (TCCs) obtained from healthy thyroid parenchyma of patients submitted to thyroid surgery for reasons different from AIT. Following exposure to amiodarone the cellular model was tested for amiodarone induced cytotoxicity, and for the expression of autophagy markers. Moreover, we tested the TCCs model for co-exposure with amiodarone and autophagy inducers/inhibitors. In vivo studies involved the implementation of a database containing the clinical data of AIT patients followed at the Endocrinology Unit of the Pisa University Hospital during the course of the previous 30 years, and the creation of an outpatient service specifically committed to the care of amiodarone-induced thyroid diseases. The collected clinical data were used for both retrospective and prospective studies. Results: Amiodarone exerted a cytotoxic effect on TCCs and the amiodarone concentration that induced the half maximal effect on apoptotic cell death was calculated (IC50) as 61.5 µM. Amiodarone exposure activated autophagy on the TCCs model, as indicated by an increase of the autophagy marker Atg7, and a decrease of the autophagy substrate SQSTM1/p62 concentrations. When TCCS were co-treated with amiodarone and the autophagy inhibitor 3-methyladenine reduced cell viability. On the contrary, co-treatment with amiodarone and the autophagy inducer rapamycin did not reach a statistically significant increase in the cell viability, but resulted in an incremental trend that requires further studies. The retrospective analysis of 48 pathology specimens from total thyroidectomies for AIT performed at our University Hospital, besides highlighting the differences between the two AIT types, revealed the presence of n important macrophage infiltration in AIT2. Although not influencing the clinical course of AIT2, in real life treatment with metformin (for type 2 diabetes, independently from the thyroid disease) resulted in a delayed development of AIT. Retrospective and prospective clinical studies on the cohort of AIT patients followed at our Department were published. Conclusions: Autophagy may play a role in the pathogenesis of AIT2 by interplaying with the cytotoxic effects exerted by the exposure to amiodarone on thyroid cells. A deeper insight in the pathogenesis of AIT might lead to the identification of novel treatment strategies.